Evaluating Pile Pinning and Shielding During Finite-Length Lateral Spread

Earthquake-induced deformation of bridge piers underlain by liquefied soil may impose significant demands on the foundation system due to lateral spread. A relatively simple equivalent-static analysis (ESA) procedure can adequately predict the response of bridge foundations to lateral spreading. Typical procedures for ESA analysis assume a 'infinite-length' lateral spread feature, implying that further increases in size would not increase the demand placed on foundations within the spread. Recent studies have indicated that if the areal extent of the spread feature is smaller than the zone of influence, the displacement demand must be reduced to account for pinning effects. This case can be considered a finite-length spread. Additionally, a reduction in demand imposed on one foundation component may occur during a finite-length spread due to the shielding effect from an adjacent foundation. This document reviews and applies a method to evaluate finite-length spread while considering pinning and shielding effects. The selected bridge system is the Taras overpass in Costa Rica. By using two-dimensional finite element analyses (FEA) interaction with ESA results, pinning and shielding effects are accounted for and compared to ESA analysis for 'infinite-length' spread. The results indicate that for bridge piers accounting for a finite-length lateral spread can significantly decrease demands on the foundations during design phases.